Complaints to police about alleged crimes linked to the use of Facebook and Twitter have increased by 780% in four years, resulting in about 650 people being charged last year, figures show.

The phenomenon of social networking crime was comparatively minor in 2008 with 556 reports made to police, according to the statistics released by 29 police forces in England, Scotland and Wales under the Freedom of Information Act. This year there were 4,908 reports in which the two sites were a factor.

Chief constable Andy Trotter, of the Association of Chief Police Officers, said the figures demonstrated a new challenge. It was important that forces prioritised social networking crimes that caused genuine harm, rather than attempting to curb freedom of expression, he said.

"It is a new world for all and we could end up in a situation where each constabulary needs a dedicated Twitter squad. In my opinion, that would not be a good use of resources in difficult financial times. We need to accept that people have the right to communicate, even to communicate in an obnoxious or disagreeable way, and there is no desire on the part of the police to get involved in that judgment.

"But equally, there are many offences involving social media such as harassment or genuine threats of violence which cause real harm. It is that higher end of offending which forces need to concentrate on."

Police forces were asked to provide the number of crime reports in which either Facebook or Twitter was a key factor. This included alleged offences committed on the sites, such as posting abusive messages, and those provoked by postings, including violent attacks.

A total of 653 people faced criminal charges over the allegations this year, according to the forces that responded. Great Manchester police charged the highest number of people, at 115.

In 2008 across all force areas, 46 people were charged in cases connected to the sites.

Some forces released a detailed breakdown of crime reports, showing a wide variety of alleged offences, with harassment and menacing messages among the most common.

For example, Tayside police received 66 reports involving the sites this year, 44 of which involved obscene or menacing messages. In Merseyside, 21 of the 76 relevant crime reports this year involved harassment. Lancashire police received reports of six threats of murder.

There were also numerous alleged sexual offences, including grooming, as well as complaints of stalking, racially aggravated conduct and fraud.

Trotter said offences can be roughly divided between those that would have been committed before the emergence of social media, albeit in a different way, and those that exist because of the online platform.

"In many ways, online communities can be self-regulating and good at weeding out unacceptable behaviour. We need to find a way of distinguishing between that type of behaviour and that which requires police intervention."